1. Field of the Invention
This invention relates to glass compositions for ophthalmic lenses having a refractive index of at least 1.69, an Abbe value of at least 39 and a specific gravity of not more than 3.2.
2. Description of the Prior Art
The rate of correction of an ophthalmic lens depends upon its refractive index and the curvatures of both surfaces thereof. When myopia or hypermetropia advances, a lens having a thicker edge portion or central portion is required in order to correct the same. This not only impairs the beautiful appearance of the ophthalmic lens, but the heavy weight thereof also makes the wearer uncomfortable.
Accordingly, ophthalmic lenses which are light in weight and which have a high refractive index have been desired, and Japanese Patent Application (OPI) 87716/74 discloses an ophthalmic lens glass which is intended for this purpose. However, the glass disclosed in this Patent Application has an Abbe value of as low as 30.0 to 31.5, and has the defect that when it is used as an ophthalmic lens, it has high chromatic aberration at its outer edge portions. Especially in the case of ophthalmic lenses having a high diopter value, their chromatic aberration is high.